This invention relates to a color cathode ray tube having an evacuated envelope with means for producing at least one electron beam, a cathodoluminescent screen and a color selection electrode adjacent to the screen with a heat dissipative and electron reflective coating on a surface of the color selection electrode. The novel coating is capable of minimizing local heating of the color selection electrode, especially in high brightness portions of the display, by backscattering a large number of the incident electrons which would otherwise cause local expansion of the color selection electrode and generate color errors. The coating also has a surface which provides effective radiative heat transfer.
During operation of the color cathode-ray tube having a color selection electrode, or shadow mask, only a small fraction of each electron beam is passed through the apertures in the shadow mask. At about the center of the mask, the masking plate intercepts all but about 18% of the beam; that is, the shadow mask is said to have a transmission of about 18%. The electrons comprising the remaining 82% of the electron beam are intercepted by the mask plate on their way to the screen. The kinetic energy of the impinging electrons is converted into thermal energy with an increase in shadow mask temperatures resulting in thermal expansion of the mask. Since the mask is usually supported by a frame of substantial mass, the temperature of the shadow mask during initial warm-up will rise more rapidly in the center than at the edge. This causes the mask to dome, so that the center portion of the mask moves toward the screen, while the edge of the mask maintains its spacing with the screen. Furthermore, when a large number of electrons impinge on a local area of the mask, to create high picture brightness, localized doming or blister warpage occurs unless temperature equilibrium in the plane of the shadow mask is reestablished sufficiently rapidly. Both blister warpage and overall doming of the shadow mask result in color errors due to electron beam misregister with the phosphor elements of the screen.
U.S. Pat. No. 3,878,428, issued to Kuziminski et al. on Apr. 15, 1975, discloses a CRT having a tailored heat transfer pattern formed, at least, on facing center surfaces of the screen and shadow mask. The hotter mask radiates heat to and through the lower temperature envelope. An absorptive black coating is formed on the center surfaces and a reflective layer is generally provided on at least a portion of the peripheries of the surfaces.
Such a coating structure addresses the problem of overall mask doming; however, the coating is ineffective for counteracting the color errors resulting from intense, localized electron impingement, which causes blister warpage, because the radiative heat transfer mechanism provided by the coating cannot rapidly reestablish thermal equilibrium.
U.S. Pat. No. 4,339,687, issued to Redington on July 13, 1982, discloses a layer of high atomic number material such as tungsten or gold deposited e.g., by vacuum evaporation, sputtering, vapor deposition, etc., on the electron gun-facing surface of the shadow mask to increase the percentage of electrons backscattered from the mask. This is disclosed to be effective for reducing the effects of localized doming or blister warpage.
Gold, of course, is economically impractical and the disclosed techniques for depositing tungsten are not cost effective or practical for large size tubes.
U.S. Pat. No. 4,442,376, issued to Van Der Waal et al. on Apr. 10, 1984, discloses a heavy metal having an atomic number exceeding 70 with a high electron reflection coefficient to minimize energy absorption by the shadow mask. Suitable materials, which are economical, include the heavy metals tungsten, lead and bismuth and their compounds selected from the group consisting of carbides, sulphides and oxides.
However, it is known to those skilled in the art that the use of sulfides within the tube envelope is generally avoided since sulfides have a detrimental effect on cathode emission. Carbides are difficult to deposit and thus add to the cost of tube manufacturing. Likewise, oxides form generally thin layers, and considerable time, and therefore additional cost, is required to provide an oxide layer of suitable thickness.
A need therefore exists for a shadow mask coating that addresses the problems of both overall doming and blister warpage, and that is inexpensive and practical to apply.